The present invention relates to an earth leakage breaker. More specifically, the present invention relates to an earth leakage breaker that facilitates avoiding unnecessary break caused by lightning surge.
FIG. 16 is a block diagram of a conventional earth leakage breaker. FIG. 17 is a time chart of the outputs from the constituent devices of the conventional earth leakage breaker of FIG. 16. Referring now to FIGS. 16 and 17, a secondary current is generated in a secondary winding of a zero-phase current transformer 3 (hereinafter referred to as a "ZCT") wound around main circuit conductors 2 of an earth leakage breaker 1 when a leakage current or a grounding current caused by lightning surge flows in the main circuit conductors 2. The secondary current is converted to a voltage signal by a resistor 4. The voltage signal is amplified by an amplifier 6 after impulsive noise or high frequency leakage current components on the secondary side of an inverter are removed from the voltage signal by a low-pass filter 5. The output of the amplifier 6 is resolved to positive and negative pulses by the threshold of a first comparator 7. The positive and negative pulses are synthesized in an OR circuit (OR1). The output of the first comparator 7 through the OR circuit (OR1) is integrated in an integrator circuit 8. When the output of the integrator circuit 8 exceeds the threshold of a second comparator 9, a trip-signal generator circuit 10 feeds a trip signal .gamma. to a trip coil 11 based on the output .alpha. of the second comparator 9 to trip the earth leakage breaker 1.
The breaking period of the high-speed earth leakage breaker is specified to be within 40 ms by the IEC specification for protecting the human body. Since it usually takes about 23 ms at the maximum for the earth leakage breaker to operate mechanically, 40-23=17 ms is left for the judging period until feeding the trip signal to the trip coil. If the period during which the output of the amplifier 6 exceeds the threshold of the first comparator 7 described in FIG. 17, i.e. the pulse width of the output of the first comparator 7, is set at 7 ms and the pulse interval at 3 ms, the total pulse width necessary to output the trip signal .gamma. within the foregoing 17 sec., i.e. the integration period of the integrator circuit 8, will be 17-3=14 ms.
In the case of the grounding current due to the lightning surge that is not to be broken, if the period during which the output of the amplifier 6 exceeds the threshold of the first comparator 7 is 5 ms for the positive signal and 20 ms for the negative signal, the total is 25 ms, which exceeds foregoing 14 ms.
Therefore, when the pulse interval is set at 2 ms, trip signal .gamma. will be outputted within 16 ms and the unnecessary break will be caused.
In view of the foregoing, it is an object of the invention to provide an earth leakage breaker that exhibits improved performances.
It is another object of the invention to provide an earth leakage breaker that does not cause any unnecessary break due to the lightning surge.
It is a further object of the invention to provide an earth leakage breaker that securely breaks the periodical short-circuit current, i.e. heavy-grounding current, as well as the usual leakage current.